The "mem" in celt_fir_c() either is contained in the head of input "x"
in reverse order already, or can be easily attached to the head of "x"
before calling the function. Removing argument "mem" can eliminate the
redundant buffer copies inside.
Update celt_fir_sse4_1() accordingly.

Avoids pre-echo in hybrid mode caused by noise being injected in the first band

This could happen when we had more than 32 bits on the first hybrid band with
a transient just in the middle of the frame. The band would be split and the
first half of the frame could end up with non-zero energy, but not enough
bits for a pulse. Because it's the first band, no folding would be possible.
This would cause noise to be injected for the entire duration of the first
half and that noise should then get folded to higher bands.

The change also makes the analysis run for sampling rates of 16 kHz and 24 kHz
since the features are only computed on the 0-8 kHz band. The longer time
window (20 ms instead of 10 ms) makes the tonality estimator more reliable
for low-pitch harmonics.

Ensure that the SILK encoder's log gain is 63, not 64, when encoding
a maximum-value delta gain index of 40. This matches the decoder
and RFC 6716 4.2.7.4, and prevents an assertion failure in the rare
case that the gain is later independently coded.

Also remove the SILK_DEBUG_STORE_CLOSE_FILES flush call from
opus_demo.

This is debugging code which is no longer used, but defining
the symbols for SILK_DEBUG_STORE_CLOSE_FILES and calling it
from opus_demo causes linking problems on Microsoft Visual Studio
where we have strict controls on public symbols and want to
test the compiled DLL.

Since the code isn't in active use, it's better to remove it
to avoid clutter and address the linking issue.

Fix Hybrid redundancy assertion failure in ec_enc_shrink(), even
if SILK produces more than maxBits, by reducing or eliminating
redundancy when necessary. Don't reserve space for redundancy that
is too small to be used for redundancy. When there is not enough
space for redundancy, allow the regular frame to use all the bits.

Different distributions of doxygen have different default
values of HAVE_DOT setting, so we need to pick a specific
setting to avoid 'missing dot' warnings on some platforms.

Doxygen uses it to generate inclusion graphs for our various
header files, which is somewhat useful, but not essential.
We therefore enable dot if it's present (usually through
the parent graphviz package) but disable it if it's not
available, silencing the warning, but not giving uniform
results.

The multistream encoder input has st->layout.nb_channels channels.
The number of channels actually encoded is st->layout.nb_streams +
st->layout.nb_coupled_streams, which may be fewer, so ideally it
would only analyze those when computing the optimal frame size, but
there is no code to do that currently.